The present invention relates generally to improved circuits and methods for generating the gamma correction voltages required for achieving satisfactory performance in driving LCD displays (liquid crystal displays), and more particularly to circuits and methods which allow reduced size and power consumption of gamma correction buffers in gamma generator systems that are used in conjunction with column drivers of LCD display systems.
The closest prior art is believed to include the assignee's pending patent application entitled “METHOD AND APPARATUS FOR SETTING GAMMA CORRECTION VOLTAGES FOR LCD SOURCE DRIVERS”, Publication No. 20060202929, Ser. No. 11/079,357 filed Mar. 14, 2005 by Baum et al. and incorporated herein by reference.
Color LCD displays are widely used for desktop computers, laptop computers, and TVs, and consist of LCD pixel elements that typically are controlled by a matrix of intersecting gate drivers (also known as row drivers) and source drivers (also known as column drivers). In “Prior Art” FIG. 1 (which is the same as FIG. 4 in the above referenced Baum et al. application), LCD display system 10 includes a LCD display panel 11 having many rows (depending on the height of LCD display panel 11) of LCD pixels selectable by lines 14 that are driven by gate driver circuitry 12 in response to signals sent by controller circuitry 32 via conductor or bus 38. LCD display panel 11 includes many columns (e.g., as many as 4096 columns are more depending on the width of the LCD display panel 11) of LCD pixels coupled, respectively, to gamma reference voltage signals produced on conductors 20-1,2 . . . q by a resistor-string DAC 16, where q is the number of columns of pixels.
The switches in source switch driver circuitry 18 are used to tap off the various voltages of R-DAC 23. The corrected gamma curve is established by programming the desired voltages along the various tap points of R-DAC 23. Then the source driver switch circuitry 18 can connect the appropriate voltages to the R-DAC outputs 20-1,2 . . . , and hence to the appropriate control terminals of the LCD display, at the appropriate times. Source driver switch circuitry 18 in resistor-string DAC 16 produces intensity or brightness control signals on conductors 20-1,2 . . . q for controlling the gray scale (i.e., the brightness or intensity of the LCD pixels in each column at its intersections with the selected rows).
The source drivers in source driver switch circuitry 18 are used to control the gray scale of each pixel by converting the digital image data 36 into corresponding voltages produced by means of resistor-string DAC 22 and multiplexing the appropriate voltages by means of the source driver switch circuitry 18 to the appropriate LCD brightness control outputs 20-1,2 . . . q to corresponding columns of pixel elements.
The gray scale transmission characteristic of resistor-string DAC 22 is typically “nonlinear” to compensate for the non-linear transmission characteristic of the LCD display 11. The nonlinear behavior of the resistor-string DAC 22 can be thought of as being represented by an “intrinsic” gamma correction curve (sometimes also referred to as a “color curve”). The nonlinear transfer function of each LCD display 11 is unique, and therefore the intrinsic gamma curve built into the source driver circuitry 16 by resistor-string DAC 22 ordinarily must be modified to achieve optimum display performance of a particular LCD display screen. The “gamma voltage correction” involves correcting the above-mentioned intrinsic gamma curve so as to make the “gray scale” of displayed LCD screen images appear more satisfactory in the eyes of a trained expert.
The string DAC resistors 23 are connected in series between a high reference voltage VH and a low reference voltage VL, and the voltages on conductors 19-1,2 . . . m generally define a corrected gamma curve. (As an example, the number of resistors is m=256 for an 8-bit source driver.)
Gamma reference voltage generator circuit 35 includes logic circuitry 30, DACs 28-1,2 . . . m and buffers 24-1,2 . . . m. (Buffers 24-1,2 . . . m also are referred to herein as “buffer amplifiers” and as “gamma correction buffers”.) Buffers 24-1,2 . . . m could be included within DACs 28-1,2 . . . m. Gamma reference voltage generator 35 is coupled by a conventional I2C bus 34 including a SDA conductor and a SCL conductor to controller 32. Outputs of logic circuit 30 are connected to the inputs of DACs 28-1,2 . . . m, the outputs of which are connected to inputs of corresponding buffers 24-1,2 . . . m, respectively. The outputs of buffers 24-1,2 . . . m are connected to conductors 19-1,2 . . . m, respectively, which may be but are not necessarily directly connected to the q inputs of source driver switch circuitry 18. The output voltage values of buffers 24-1,2 . . . m are determined by the reference voltages VH and VL and by the value of the binary input code (not shown) used to “program” that buffer.
Logic circuit 30 operates in response to data and clock signals received on I2C bus 34 from controller 32 and performs the function of assembling the digital inputs for DACs 28-1, 2 . . . m so as to produce desired gray scale or intensity of pixels in the row currently selected by gate drive circuitry 12 in response to digital gray scale codes received from either an internal non-volatile memory of the controller 32 or from an external EEPROM and converted to the digital signals that are applied to the inputs of the various DACs.
Gamma correction buffers 24-1,2 . . . m must supply most of the correction currents from buffers that are almost midway between the power supplies VH and VL. This is the worst case for power consumption in the gamma correction buffers. LCD manufacturers have been concerned about this problem for some time and desire a solution that will reduce the power and the size of gamma correction circuitry for state-of-the-art LCD display systems. The various competitors in the field are believed to be working on various ways of reducing the above mentioned power dissipation.
Perhaps this can be understood by referring to “Prior Art” FIG. 2, wherein the DAC/buffers 35-1 each have an output coupled to a corresponding voltage tap point of a resistor string 23-1. The resistor string can be part of a single R-DAC 23 as shown in FIG. 1. More typically in state-of-the-art LCD display systems, the resistor string can be divided into multiple resistor strings which are included in R-DACs (resistor DACs), respectively, such as “upper R-DAC” 23-1 and “lower R-DAC 23-2”, as also shown in subsequently described FIG. 3A. (Whether the resistor string is divided into multiple resistor strings which are included in multiple R-DACs, respectively, depends on the LCD panel that is to be driven by the gamma curve circuitry.)
In FIG. 2, the various illustrated sinking currents and sourcing currents, i.e., the gamma correction currents, flowing into and out of the output terminals of the various gamma correction buffers 24-1,2 . . . 22 all are determined by the values of digital input signals (not shown) which are programmed into the corresponding DACs 28-1,2 . . . 22, depending on the particular LCD panel to which the gamma correction buffers are connected in order to correct the intrinsic gamma curve of that LCD panel. Note that the values of resistors R1,2 . . . 20 are shown in FIG. 2. The gamma correction currents flowing in the gamma correction buffer output conductors 40-1,2 . . . 22 also are shown in FIG. 2. The tap voltages V0,1,2 . . . 21 have been digitally programmed into the corresponding DACs 28-1,2 . . . 22 and produced in analog form by the corresponding gamma correction buffers 24-1,2 . . . 22. The tap voltages V0,1 . . . 20 represent the corrected gamma curve of LCD display panel 11, and are illustrated along the right sides of resistor strings 23-1 and 23-2 in FIG. 2.
In the example of FIG. 2, all of the gamma correction buffers 24-1,2 . . . 22 are powered by VDD and ground, where VDD is 18 V. Whether a particular one of gamma correction buffers 24-1,2 . . . 10 and gamma correction buffers 24-13 . . . 22 operates to source a gamma correction current or to sink a gamma correction current depends on the corresponding digital input gamma correction current value which has been programmed into its corresponding DAC. In this example, the top terminal voltage V0 of upper resistor string 23-1 is a 16.4 volt output of buffer 24-1 and has been programmed by a digital input 26 (FIG. 3B) to DAC 28-1. V0 is 1.6 volts below VDD, which is 18 volts. In this example, a 2.408 milliampere current flows into the output terminal of buffer 24-11, which is the bottom buffer in upper R-DAC 23-1.
In a typical LCD display system, a corresponding gamma correction buffer typically would be included for each of 10 or more R-DACs. If the LCD display system having the intrinsic gamma curve represented by the values of resistors R1,2 . . . 20 indicated in FIG. 2 has 10 upper and lower R-DACs, the total gamma correction current into the outputs of the 10 upper R-DAC buffers 24-11 would be 24.08 mA. This current would flow through a voltage drop of V10=8.613 volts in the buffer output transistors absorbing that current, resulting in a undesirably large amount of power dissipation of 207.4 milliwatts. Also, a current of 2.562 mA would flow out of the buffer output terminals of each of the 10 lower R-DAC buffers 24-12. This total current of 25.62 mA would flow through a voltage drop of VDD minus V11, i.e., 18-8.41=9.59 volts, and in the buffer output transistors delivering that total current, resulting in another undesirably large amount of power dissipation of 245.7 mW.
Note that power dissipation in the other buffers 24-1,2 . . . 10 and buffers 24-13 . . . 22 is much lower than in the two “middle” buffers 24-11 and 24-12 because the two “midrange” gamma correction buffers 24-11 and 24-12 sink and source, respectively, the total current through the upper resistor string 23-1 and the lower resistor string 23-2, respectively. However, the other gamma correction buffers sink or source gamma correction currents which are much smaller.
Thus, there is an unmet need for a gamma correction current circuit and method which substantially reduce the amount of power dissipated in an LCD display system.
There also is an unmet need for a gamma correction current circuit and method which substantially reduce the physical size of output transistors in the buffers thereof.